Republican presidential candidates Marco Rubio, left, and Jeb Bush speak at the Iowa Republican Party's Growth and Opportunity forum, a festival held at the Iowa State Fairgrounds in Des Moines on Saturday.

DES MOINES, Iowa — Neither Jeb Bush nor Marco Rubio emphasized the need to do well in Iowa in the early months of the 2016 presidential campaign. But that has changed as their rivalry intensifies and pressure mounts to emerge from the Feb. 1 caucuses as the favorite among mainstream conservatives.

Neither has visited Iowa as often as more socially conservative candidates such as Texas Sen. Ted Cruz, and both lack the star power of rivals Donald Trump and Ben Carson.

But the key, unveiled by Bush in a donor conference and reinforced after Wednesday’s debate, is how they fare against each other — and Iowa is the first test.

“They’re competing for the same segment of Iowa Republicans,” said former state party chairman Matt Strawn, who said that vote still “absolutely is up for grabs.”

As a result, they are adding resources in the state and spending more time courting voters.

Bush and Rubio were among 10 presidential candidates to appear at the Iowa Republican Party’s Growth and Opportunity forum, a tailgate-themed festival at the Iowa State Fairgrounds in Des Moines, and the last multicandidate event before the caucuses.

Bush, a former Florida governor, was seen as a potential front-runner based on his early fundraising success. But he has failed to catch fire since summer.

Meanwhile, the younger Rubio has called for a new generation of leaders but has struggled to grab the national limelight.

But at the Des Moines forum on Saturday, Rubio tried to capitalize on a strong showing at Wednesday night’s national debate and an increasing curiosity about him.

Rubio told more than 1,000 Iowa GOP faithful that the nation is on the wrong path and assured them there “is a road that will allow us to be the freest and will allow us to leave our children better off,” he said. “To do that, we must turn the page and allow new leaders and new principles.”

The jab at Bush, the 62-year-old son and brother of former presidents, is more subtle than Bush’s now outright battle with Rubio for missing Senate votes while campaigning for the White House, a criticism Bush raised during Wednesday’s debate.

On Saturday, Bush again mentioned Rubio’s Senate attendance, with an Iowa twist: “If you’re elected to serve, you should do what Chuck Grassley does: You should show up to vote,” Bush said, referring to Iowa’s senior senator.

To date, the better-funded Bush has invested more and earlier in a political organization, with at least 10 staffers in the state, as well as having as a senior national adviser David Kochel, a veteran Iowa GOP operative.

Rubio, who has had a leaner operation, has 30 people working across the first four nominating states — Iowa, New Hampshire, South Carolina and Nevada.

“I think Bush has an organizational advantage,” Iowa Republican chairman Jeff Kaufmann said. “But I think Rubio has a momentum advantage. Also, I think he has an advantage coming out of the debate.”

Now Rubio’s fundraising is also rising.

Billionaire investor Paul Singer on Friday announced his support for Rubio in a letter to his extensive network of Republican fundraisers, encouraging them to follow his lead.

Bush’s team views Rubio as his most dangerous competitor for voters and donors who want to see a traditional nominee. Wednesday night’s attack from Bush backfired.

During a post-debate conference call, Bush’s team told supporters the debate did not go well and tried to reassure skittish donors the turnaround loomed ahead.

After his speech, Rubio was mobbed by supporters and journalists. He shook hands, signed books and posed for photos for an hour, joking easily with the crowd. “Just make sure you’re here in February,” he said to a woman who told him she spends time in Florida.

Bush’s stand, decorated by hay bales, offered cotton candy.

“I have enough humility to know that I’ve got to get better,” he told reporters. “I’m going to get better. I have enough self-awareness to know that.”

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